1. Field of the Invention
The invention relates to a non-volatile semiconductor memory.
2. Description of the Prior Art
An EPROM (Erasable and Programmable Read Only Memory) is a non-volatile rewritable read only memory. In the EPROM, data is stored by accumulating charges into a floating gate and data is erased by escaping the charges accumulated in the floating gate to the outside by irradiating ultraviolet rays.
To realize a large capacity of the EPROM, in addition to the realization of a high integration density of memory cells by reducing an area of each memory cell, it is necessary to accomplish both the improvement of the writing characteristic and the improvement of the reading reliability by the improvement of the soft write (phenomenon such that a writing operation gradually occurs upon reading) resistance.
As a prior art literature of the invention, there can be mentioned the literature of ISSCC 80, page 146 which disclose an EPROM having what is called an X cell structure in which memory cells are arranged like an X-character.
When data is written in the EPROM, a program voltage V.sub.pp is applied to a control gate and a drain region and hot carriers generated by an avalanche breakdown or hot carriers generated in a channel portion are injected into a floating gate, thereby changing a threshold voltage V.sub.th of a memory transistor.
In the EPROM, on the other hand, when data is read out as well, a voltage is applied to the control gate and the drain region. Therefore, even when data is read out, hot carriers generated in portions near the drain region are likely to be injected into the floating gate. Particularly, in the case of using the EPROM for a long time, consequently, there is a problem such that the soft write occurs and a fluctuation of the threshold voltage V.sub.th of the memory transistor is caused and the reading reliability deteriorates.
To improve the soft write resistance, a method whereby the memory transistor is formed so as to have an LDD (Lightly Doped Drain) structure is considered. If the above method is used, however, since a field concentration near the drain region is weakened, the hot carriers are hardly injected into the floating gate. Thus, there is a problem such that the writing characteristic deteriorates.